1. Field of the Invention
The present invention relates to a control device for a tool driving unit such as a tapping unit for machining a workpiece by means of a tool, and more particularly to a control device for a tool driving unit wherein a tool such as a tap, drill, or reamer which is held by a chuck mounted at a front end of a spindle is controlled to be advanced or retracted while being rotated.
2. Discussion of the Related Art
A conventional tool driving unit such as a tapping unit or a drill unit has a tool such as a tap or drill which is held by a chuck mounted at a front end of a spindle. A rotational driving mechanism is provided for rotating the spindle around a central axis thereof, and a feed driving mechanism is provided for longitudinally moving the spindle along its central axis so that the spindle is advanced or retracted while being rotated. In this conventional tool driving unit, a single motor is employed to power both the rotational driving mechanism and the feed driving mechanism. A torque is transmitted from an output shaft of the motor through the rotational driving mechanism which includes, for example, gears and belts to a spindle so as to rotate the spindle. Torque is converted into a feed driving force by the feed driving mechanism which includes, for example, a ball thread or a rack and pinion to move the spindle longitudinally along its central axis, thereby carrying out a desired tapping or drilling operation.
However, such a conventional tool driving unit is relatively large due to the size of the rotational and feed driving mechanisms, and has a large number of parts constituting the torque transmitting mechanisms. Additionally, a mechanical backlash tends to occur in the feed driving mechanism, causing a reduction in position accuracy for machining. Further, when replacing one tool with another, it is necessary to replace portions of the transmitting mechanisms, such as gears, as well as the tool, so as to obtain operating speeds suitable for different tools used therewith. For example, when performing a drilling operation, the spindle is rotated at a high speed and moved longitudinally at a relatively low speed, while when performing a tapping operation, the spindle is rotated and moved longitudinally at relatively low speeds. Thus, it is difficult to reduce a cost of operating the tool driving unit.
Another conventional tool driving unit has, as disclosed in U.S. Pat. Nos. 4,813,821, and 4,692,071, an arithmetic unit which acts as a control device for controlling the tool driving unit, a position detecting/control unit, and a speed detecting/control unit for controlling the speed and position of the spindle. The control device for the tool driving unit, the position detecting/control unit, and the speed detecting/control unit for the spindle are usually constructed from analog circuits. Accordingly, there is a possibility of deterioration and offset of an operational amplifier used therewith over time, for example, and it is therefore necessary to individually adjust control settings of the control system.